The ever-increasing requirements of today's optical-network systems require a sensitive device to accurately receive data and measure optical power. As the requirements for high data rates grow, more and more systems will transition to high-speed and high-sensitivity optical receivers. The designers of these optical-network systems have many types of optical receivers from which to choose. One such choice for a fast and highly sensitive optical receiver is an Avalanche Photo Diode (“APD”).
APDs differ from other photodiodes in that APDs can provide gain, meaning that the ratio of incoming photons to outgoing electrons is greater than 1:1. This ratio, or avalanche multiplication factor, is controlled by a reverse bias voltage applied across the APD. APDs provide significant advantages over other types of optical receivers. Of particular note is that APDs exhibit low noise, broad spectral and frequency responses, and have high gain ranges provided by their internal photo-electronic signal gains.
The difficulty with using APDs is their high cost and their vulnerability to damage due to high-levels of received optical power. For example, APDs are often used in laser-based optical-fiber systems, such as wave division multiplexed (“WDM”) networks. The Erbium-Doped Fiber Amplifier (“EDFA”) is a device in the WDM network that boosts the signal in the optical fiber. However, the EDFA amplifies all channels in a WDM signal simultaneously, so as wavelength channels are added or dropped a high optical power spike can occur on one of the newly added or remaining channels. This high optical power spike causes a rapid increase in total current that flows through a biased APD and thereby may cause permanent damage to the device.
To maintain reliability of the optical network, the optical receiver should be able to continue to operate, without damage, after exposure to periods of high optical power. To operate continuously, rapid and effective safety mechanisms should be in place to detect such high optical power conditions and correspondingly protect the optical receiver.